Spleen tyrosine kinase (Syk) is an intracellular protein-tyrosine kinase that is widely expressed in hematopoietic cells, cells of the immune system and cells of epithelial lineages. Syk is involved in transducing signals from activated immunoglobulin receptors, such as Fcγ receptors and Fcε receptors. Syk is also implicated in the signaling triggered by receptors for G-CSF, IL-2, IL-3, IL-5, GM-CSF, IL-15, various chemokines, IL-1 and erythropoietin. Thus Syk is a promiscuous messenger in various signal transduction pathways. Syk mediates diverse cellular responses, including proliferation, differentiation, and phagocytosis.